1. Field of the Invention
This invention relates to a ferroelectric memory device including a BiFeO3 ferroelectric layer.
Priority is claimed on Japanese Patent Application No. 2003-173247, filed Jun. 18, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
Ferroelectric materials are beginning to be put to practical use as recording materials in nonvolatile memory devices because they have a large capacity and low power consumption. Pb(Zr1-xTux)O3 (PZT) which is a perovskite-type oxide and SrBi2Ta2O9 (SBT) which is a Bi layered compound are representatives of ferroelectric materials used in nonvolatile memory devices. For a nonvolatile memory device using such ferroelectric materials, namely a ferroelectric memory device, it is expected that its dielectric polarization moment and Curie temperature (transition temperature from paraelectric to ferroelectric), which are characteristics of the device, are large and high.
For example, it is expected, for better sensitivity of a sense amplifier, that its residual dielectric polarization moment Pr is at least larger than 10 μC/cm2. Especially, the dielectric polarization moment is expected to be even larger for a memory device with a higher density including a capacitor with a smaller area than that of capacitors in existing devices. From the viewpoint of reliability of recorded data, the Curie temperature is expected to be more than 200° C. Especially, recording materials are expected to be able to hold memory near the operating temperature range (e.g., from −10° C. to +100° C.), that is, it is expected that structural phase transition does not occur in the temperature range of the recording materials. Additionally, it is necessary that the materials can record and reproduce data repeatedly more than 1012 times, preferably 1015 times. From the viewpoint of future miniaturization, the thickness of the recording film (ferroelectric layer) is expected to be 50-200 nm, followed by 10-50 nm. In this case, leak current through the ferroelectric layer is expected to be as low as 10−8-10−6 A/cm2 at the time of a 100 kV/cm application.
Ferroelectric materials belonging to the PZT family exhibits a large dielectric polarization moment, e.g., 30-50 μC/cm2, and have a Curie temperature of more than 400° C. and therefore the structural phase transition does not occur in the operating temperature range of the materials. However, composition control of Pb is difficult because Pb can easily evaporate from the ferroelectric layers. Additionally, the evaporation of Pb leads to deterioration of the environment because Pb is harmful thereto. For these reasons, production of ferroelectric memory devices including these materials needs to be reviewed.
Ferroelectric materials belonging to the SBT family exhibits a dielectric polarization moment of 20 μC/cm2 at maximum. However, its orientation control in the materials is difficult because of a layered structure, and miniaturization of devices made of these materials is difficult because of the existence of crystal grains in the materials. Additionally, the materials are damaged by hydrogen at the time of formation of a passivation film in a post-process. Under these circumstances, BaTiO3 is also one promising ferroelectric material. This material exhibits a dielectric polarization moment of 30 μC/cm2 along the c-axis at room temperature. BaTiO3 has a low Curie temperature of 120° C. and therefore structural phase transition from the tetragonal phase to the orthorhombic phase occurs near 0° C. in the materials. Since BaTiO3 has such a transition temperature near the operating temperature, the value of the dielectric polarization moment is unstable and the materials may degrade easily with the structural phase transition.
In light of these circumstances, BiFeO3 is proposed as a new ferroelectric material for a ferroelectric memory device (see, e.g., Japanese Unexamined Patent Application, First Publication No. 2001-210794). It has been confirmed in recent reports that BiFeO3 has a high ferroelectric characteristic of dielectric polarization moment of 60-70 μC/cm2 (see, e.g., Science, Vol. 299, 1719-1721, 2003). In the article in Science, upper and lower electrodes between which a ferroelectric layer is sandwiched are made of SrRuO3, and the ferroelectric layer made of BiFeO3 has a perovskite structure in a tetragonal system.
Recently the demand for high performance and high-density integration of semiconductor devices is increasing. Accordingly, the high performance is demanded also for a ferroelectric memory device, and additionally, directly mounting onto a Si substrate or implementation to a Si substrate is expected. For this reason, BiFeO3 is thought to be a very promising candidate for a ferroelectric material because it has a high ferroelectric characteristic as described above, and does not contain Pb that is detrimental to the environment.
However, there is a problem to be solved in a ferroelectric memory device using BiFeO3, that is, it is very difficult to directly implement the ferroelectric memory device including a lower electrode on an Si substrate. This is because normally a natural oxide film is formed on an Si substrate, and it is difficult to make a lower electrode film and a BiFeO3 film grow epitaxially on the natural oxide film. For this reason, under the existing circumstances, implementation of a ferroelectric memory device using BiFeO3 with a tetragonal structure on an Si substrate has not been realized yet.
In view of these circumstances, the purpose of the present invention is to provide a ferroelectric memory device which has high performance and does not contain Pb and which is directly implementable on an Si substrate.